Good grief who cares? Some introductory text in the dm advice book’s section on how magic works in different worlds doesn’t
matter. Even what the game’s “default” is or isn’t
does not matter in any way.
Now, can you explain why you think this line of argument is even relevant are a response to an argument that the worldbuilding assumptions of very-rare-magic worlds don’t even make internal sense?
Goodness gracious, Max. I know exactly how spell books work. That is literally the point of what you quoted.
Though if we are being pedantic, and I’d rather we not be, spellbook a are written in an idiosyncratic semi-cypher, which is why you have to rewrite spells in your own idiosyncratic methodology in order to copy them.
Not that it matters, because the entire point of what you quoted was making a statement about how Spellcasting
doesn’t work under the rules!

what!? Prove a subjective value judgement!? What on earth are you even talking about?
I’m not making anything out to be anything, in fact. I’m challenging the assertion of others that it makes perfect sense to have it be extremely rare.
Thank you. It was…somewhat aggravating to have the claim repeated without a quotation or at least page reference.
Let’s explore the rest of the section of that chapter you’re quoting, however.
Now, I can’t quote pages, because DDB inexplicably doesn’t list them so that you can search by them or reference them again in a physical book, and I only own physical copies of books as collection items.
However, I can just take screencaps.
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Okay, so what is stated “unequivocally” here is actually that the rarity of magic is variable. That’s what “most” means.
And then it describes a world where magic isn’t really even actually rare, it’s just…inexplicably concentrated in places where people gather, and never proliferates…because reasons.
The section also switches from speaking of generic hamlets with no specified world to using a counter-example of the Forgotten Realms, where there are mages guilds.
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This presents a multiverse without a default. The closest it comes to a default is a very vague “most worlds”, which inherently means not all. Not “nearly every” or the “overwhelming majority”, just “most”.
The only really strong default is that the book assumes every world
has some magic. Everything else varies.
None of which makes the worldbuilding sections of the dmg particularly helpful in a discussion about what makes sense in a D&D world. Especially compared to the options in the book and what their presence implies about worlds in which they exist.